Why does time slow down when you travel near the speed of light?

Question

We learned about the Einstein train that travel nearly the speed of light and it's just said that the time slows down when you get close to the speed of light. But I don't understand how did they come to this conclusion from that example or any other example?

Answer 1

Hello Edona -

 

Everything in relativity is about reference frame. If you are on a train platform, then that is your reference frame. A passenger going by on a train is in a different reference frame. So we talk about measurements made in our reference frame and measurements made by observing other reference frames.

 

What relativity says is that time as measured in reference frames different from ours flows at a different rate. This means for example, that if a train goes by me on the platform, and I were a to be able to measure time, from the platform, on the train, then I would say the clocks on the train were running slower than mine. 

 

This is really just a consequence of the speed of light being constant for all observers. If time is measured by light pulses in a clock, for example, then the platform person sees the pulses traveling further (the hypotenuse of a right triangle) than the person in the train sees them. So a tick on the train, AS VIEWED FROM THE PLATFORM, is slower than as viewed on the train, since the light travel distance is longer.

 

The two postulates of relativity are that light is a constant in all reference frames and that there is no preferred reference frame. So the train traveler would say the clocks on the platform would run slower than their clock. Hence the twin-paradox, which points out the difficulty of resolving who is "right". That is another question, perhaps, though it does have an answer, as Quang has pointed out.

 

There are somewhat simple equations governing the exact time changes. But check this out: police radar, using signals traveling at the speed of light, require relativistic corrections to get the right speed of a driver (another reference frame). There is no doubt the effect is real. 

 

I know this was a bit long, but I hope it helps.

 

Ken

 

Answer 2

I don't really understand time dilation completely, if at all, but the way I understand it, they found out just because the math works out. They proved it later when some scientists set two atomic clocks (which are super accurate) in sync with each other, kept one on the ground, and sent the other one on a plane ride. When they later reviewed the two clocks, they saw that the clock that was on the plane was behind the one that was on the ground. If you want there are videos on Youtube. I suggest you start with this one: https://www.youtube.com/watch?v=Cxqjyl74iu4